In an information recording medium, such as a CD-ROM (Compact Disc-Read Only Memory), a CD-R (Compact Disc-Recordable), a DVD-ROM, a DVD-R, a DVD-RW, and a DVD+R, for example, as described in patent documents 1 and 2 or the like, there is also developed an information recording medium, such as a multilayer type or dual layer type optical disc, in which a plurality of recording layers are stacked or bonded on the same substrate. Then, on an information recording apparatus, such as a DVD recorder, for performing the recording with respect to the dual layer type, i.e., two-layer type, optical disc, laser light for recording is focused on a recording layer located on the front (i.e. on the closer side to an optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L0 layer”, as occasion demands) to thereby record information into the L0 layer in an irreversible change recording method by heat or a rewritable method. Moreover, the laser light is focused on a recording layer located on the rear of the L0 layer (i.e. on the farther side from the optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L1 layer”, as occasion demands), through the L0 layer or the like, to thereby record information into the L1 layer.
When information is recorded into the L1 layer, which constitutes such a two-layer type optical disc, the optimum recording power of the laser light with which the L1 layer is irradiated through the recorded L0 layer, as shown in FIG. 18(a), is 44.5 (mW: milliwatt) at which a jitter value is minimum on a parabolic curve in a thin line (with white triangles) in FIG. 18(c), for example. On the other hand, the optimum recording power of the laser light with which the L1 layer is irradiated through the unrecorded L0 layer which has a different light transmittance from that of the recorded L0 layer, as shown in FIG. 18(b), is 46 (mW: milliwatt) at which a jitter value is minimum on a parabolic curve in a thick line (with black triangles) in FIG. 18(c), for example. Thus, there is a need to consider whether or not the L0 layer is recorded, in the case of the recording in the L1 layer. With respect to this, there is devised or invented a recording method in which a so-called recording order is satisfied, which is that the laser light for recording which has penetrated the L0 layer in the recorded state is to be irradiated, for example.
However, in producing such a two-layer type information recording medium, the L0 layer and the L1 layer are formed by different stampas, and are bonded or stacked. Thus, there is a possibility to cause an eccentricity due to a bonding error, in the L0 layer and the L1 layer. Alternatively, since the L0 layer and the L1 layer are formed by different stampas, there likely arises deviation in a track pitch in each recording layer, or there likely arises deviation, a so-called dimensional error, in an absolute radial position with respect to a reference address in each recording layer. These cause a shift in the radial position of a recording area in the L1 layer which is associated with a recording area in the L0 layer by address information, such as a pre-format address, for example, and thus there arises a possibility that the above-mentioned recording order is not necessarily satisfied.
More specifically, it is assumed that the recording is performed with a recording power which is optimized in the recording after the penetration of the recorded L0 layer. When the information is recorded into the L1 layer, as shown in a left part of FIG. 19, if the laser light for recording which has penetrated the L0 layer in the recorded state is irradiated on a single track, the amplitude of a reproduction signal is large, and good signal quality is obtained. In other words, an asymmetry value, which is one example of the signal quality, is appropriate. On the other hand, as shown in a right part of FIG. 19, if the laser light for recording which has penetrated the L0 layer in the unrecorded state is irradiated, the amplitude of a reproduction signal is small, and good signal quality is not obtained. In other words, the asymmetry value is not appropriate. On the other hand, as shown in a middle part of FIG. 19, if the laser light for recording which has penetrated the L0 layer in which the recorded area and the unrecorded area are mixed is irradiated on a single track, the amplitude of a reproduction signal varies depending on the extent of an eccentric amount or a radial run-out. In other words, the asymmetry value transits from one to the other out of the appropriate level and the inappropriate level.
In order to eliminate the deviation of the optimum recording power due to the relative shift, if a recording apparatus detects the recording state of the recording area in the L0 layer which is associated with the recording area in the L1 layer, a recording control process becomes complicated because it is necessary to accurately recognize the above-mentioned relative shift. On the other hand, if the information is recorded in disregard of the deviation of the optimum recording power due to the relative shift, the control becomes complicated; for example, a process parameter for obtaining a binary signal is to be dynamically changed, on a reproducing apparatus for reproducing the recorded information, which increases a load in the reproduction process.
Thus, the inventors of the present invention propose an information recording medium in which it is possible to define an error between the address information and the radial position caused by the above-mentioned relative shift to be within tolerance.
In general, in order to measure the relative shift caused on the two-layer type optical disc, i.e., the error between the radial position at one address in the L0 layer and the radial position at another address in the L1 layer which is associated with the one address, there are two methods devised or invented as follows.
The first method is to measure the radial position of an optical head (hereinafter referred to as a “PU: Pickup”, as occasion demands) for irradiating and receiving laser light. In this method, it is possible to detect the radial position associated with a desired address, on the basis of the rotation center position of the disc and the relative position of the PU, while the desired address is read by the PU in each of the L0 layer and the L1 layer. More specifically, a position measuring device, such as a position sensor provided with the laser light and an encoder, measures the position of the PU main body, or the position of an objective lens for focusing the laser light. Moreover, by analyzing the measured position of the objective lens in time series, it is possible to analyze the state that the track of the L0 layer and the track of the L1 layer are rotated with an eccentricity. As described above, according to the first method, it is possible to measure the radial positions at an arbitrary address in the L0 layer and the L1 layer, highly accurately.
The second method is to use a microscope which can measure two-dimensional coordinates. In this method, a track on the disc is observed under the microscope, and the plane coordinates of three arbitrary points on the substantially circular track are measured, to thereby detect the center coordinates and the radius of the track. More specifically, it is possible to detect center coordinates “OL1” from three points (“Q1” to “Q3”) shown in FIG. 20. Moreover, it is possible to detect a relative position relationship of two tracks, by performing this coordinate measurement in the L0 layer and the L1 layer. The distance “d” shown in FIG. 20 allows the detection of the eccentricity in the radial direction between the L0 layer and the L1 layer. In particular, pre-recording onto the track including a desired address facilitates the detection of the track in this coordinate measurement. In the second method, it is possible to easily measure the radial position at an arbitrary address, as in the first method. In addition, the center coordinates and the radius of a track in each layer can be simultaneously detected, so that it is possible to directly understand the relative position relationship.
Patent document 1: Japanese Patent Application Laid Open NO. 2000-311346
Patent document 2: Japanese Patent Application Laid Open NO. 2001-23237